The present invention relates to a color image processing apparatus and a conversion method for converting a first color-separated signal particularly in the color image processing apparatus into a second color-separated signal.
Currently, as a conventional apparatus for visualizing and outputting a color imager a color CRT monitor apparatus, a color hard copy apparatus, or the like is known. The former apparatus intensity-modulates the light emission levels of phosphors of three colors, i.e., R, G, and B on a tube surface, and forms a visual image by additive color mixture. The latter apparatus forms a color image on paper by subtractive color mixture using Y (yellow), M (magenta), C (cyan), and K (black) coloring materials.
The above-mentioned two image display apparatuses have different color reproduction performances in principle, and the color reproduction range of the CRT apparatus is normally wider than that of the hard copy apparatus, as shown in FIG. 3.
Therefore, when two colors exist at points A and B in a certain image, if this image is displayed on the CRT apparatus, these colors are produced as different colors. However, if this image is output as a hard copy, since both the colors at the points A and B are reproduced as a color at a point C, the two colors cannot be distinguished from each other. As a result, information of an original image is lost.
Thus, when an input color image is output as a hard copy, a method in which chrominance signals in the image are converted to fall within the color reproduction range of the hard copy, and the converted signals are output may be adopted. More specifically, when chrominance signals are converted, so that the point A becomes a point D, and the point B becomes a point E in FIG. 3, colors on the hard copy can be distinguished from each other. However, since various methods of such conversion may be used, an output image may become unnatural as a whole depending on the conversion methods.
In order to solve such a problem, the following method of conversion of an inputted color image has been proposed. In this method the chrominance signal information included in the inputted image is mapped in the color reproduction range of the hard copy according to a matrix conversion, while preserving the primary colors of the color image (red, green, blue, cyan, magenta, yellow). In this case, however, reproduction of an achromatic color (gray) cannot be guaranteed (an achromatic color on the CRT apparatus cannot become an achromatic color on the hard copy).
A color having the highest saturation is detected with reference to the distribution of chrominance signals included in an input image, and the detected color is mapped in the color reproduction range of the hard copy apparatus. However, upon detection of the saturation, an input color image is evaluated on an RGB signal space. However, since the RGB signal space does not include nonlinearity of visual sense characteristics of man, even when a color having the highest saturation is detected, the detected color is not always a color having the highest saturation obtained when it is actually observed by human eye.